1. Field of the Invention
This application is concerned with multicarrier modulation techniques, which serve to transport information over a communications channel by modulating the information on a number of carriers, typically known as sub-channels.
2. Description of Related Art
Of particular interest are discrete systems where, rather than modulating a carrier with a continuously variable information signal, successive time periods ("symbols") of the carrier each serve to transmit one piece of information; that, is, the information does not vary during the course of a symbol.
Of the most practical interest is the situation where the information to be sent is in digital form, so that each symbol serves to transport a number of bits, but this is not in principle necessary and sampled analog signals could be sent--i.e. the information signal is quantized in time but may or may not be quantized in amplitude.
Quadrature modulation may if desired be used, where both the phase and amplitude of the carrier are varied, or (which amounts to the same thing) two carriers at the same frequency but in phase quadrature may each be modulated independently. A "multicarrier symbol" may thus consist of a time period during which are transmitted (say) 256 carries at different frequencies plus 256 carriers at the same set of frequencies but in phase quadrature. For digital transmission, up to 512 groups of bits may be modulated onto these carriers. Normally the carriers are harmonically related, being integer multiples of the symbol rate. This form of modulation is particularly attractive for use on poor quality transmission paths, since the number of bits allocated to each carrier can be tailored to the characteristics of the path, and indeed carriers may be omitted in parts of the frequency spectrum in which quality is especially poor.
The number of bits sent on each sub-channel may if desired be varied adaptively depending on the signal and noise levels in each sub-channel. This can be a particular advantage for transmission paths which suffer crosstalk or radio frequency interference, since the system can adapt automatically to avoid regions of frequency spectrum that are unsuitable for data transmission.
Multicarrier modulation has been standardized for use on copper pair links in a form known as discrete multitone (DMT) modulation. This is described in the technical literature (see, e.g. "Multicarrier Modulation for Data Transmission: an Idea whose Time has come", J. A. C. Bingham, IEEE Comms. Magazine, May 1990, pp. 5-14) and in a draft ANSI standard (T1E1.4/94-007) for asymmetrical digital subscriber loop technology. It is also of interest for use at higher rates than specified in the standard for use over shorter paths.
The systems referred to above may simply output successive symbols continuously to line, as illustrated in FIG. 1A; the effect of the modulation on the frequency spectrum of the output is that of a rectangular window and causes spreading (according to a sinc function) of the subchannel energy into the regions occupied by adjacent subchannels. However if the carriers are harmonically related to the reciprocal of the receiver's window durations the zero-crossings of the sinc function lie at the adjacent carrier frequencies and inter-subchannel crosstalk is avoided.
Of concern with transmission by these type of modulation over cables such as twisted pair copper is the impact of narrowband interference, especially when using a large total bandwidth (e.g. up to 10 MHz). For example, cable terminations to domestic premises may collect interference from nearby amateur radio stations (in the UK there are three amateur radio bands within the range 1-10 MHz). Of equal concern is the radiation of interference by the multicarrier transmission.
As mentioned earlier, these problems can be mitigated by not using those subchannels that are at frequencies known to lie within a band where problems of this kind occur or are expected to occur. However the improvement obtained is limited because there will still be some radiation in the band from subchannels lying outside the band, due to the spreading referred to above, and similarly receivers decoding those adjacent channels have to pick up this energy and will thus pick up some interference from the band of concern. The sinc function implies that the roll-off of amplitude as one moves away from the carrier is proportional to the reciprocal of the frequency offset.
One aim of the present invention, at least in its specific embodiments, is to alleviate this problem.